package slider components into its own widget

src/synth/Envelope.h

@@ -21,6 +21,7 @@ public:
 
     // setters
     void setSampleRate(float sampleRate) { sampleRate_ = sampleRate; }
+    void set(float a, float d, float s, float r) { setAttack(a); setDecay(a); setSustain(a); setRelease(a); }
     void setAttack(float seconds) { attack_ = std::max(seconds, 0.0001f); }
     void setDecay(float seconds) { decay_ = std::max(seconds, 0.0001f); }
     void setSustain(float level) { sustain_ = level; }